Hello.
The answer is 12/20. Even though it is a smaller fraction, it is not in simplest terms yet. A number that goes into both is 2. Divide numerator and denominator by 2 to get 6/10. But it is still not in simplest terms. 2 again goes into both numbers. Divide numerator/denominator by 2 to get 3/5. This is the simplest terms, and is greater than 5/8.
